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CHAPTER
LANDFORMS AND THEIR
EVOLUTION
A
fter weathering processes have had a part of the earth’s surface from one landform
their actions on the earth materials into another or transformation of individual
making up the surface of the earth, the landforms after they are once formed. That
geomorphic agents like running water, ground means, each and every landform has a history
water, wind, glaciers, waves perform erosion. of development and changes through time. A
It is already known to you that erosion causes landmass passes through stages of development
changes on the surface of the earth. Deposition somewhat comparable to the stages of life —
follows erosion and because of deposition too, youth, mature and old age.
changes occur on the surface of the earth.
As this chapter deals with landforms and What are the two important aspects of
their evolution ‘first’ start with the question, the evolution of landforms?
what is a landform? In simple words, small to
medium tracts or parcels of the earth’s surface
are called landforms. RUNNING WATER
In humid regions, which receive heavy rainfall
If landform is a small to medium sized
running water is considered the most important
part of the surface of the earth, what is a
of the geomorphic agents in bringing about
landscape?
the degradation of the land surface. There are
two components of running water. One is
Several related landforms together make
overland flow on general land surface as a
up landscapes, (large tracts of earth’s surface).
sheet. Another is linear flow as streams and
Each landform has its own physical shape,
size, materials and is a result of the action of rivers in valleys. Most of the erosional landforms
certain geomorphic processes and agent(s). made by running water are associated with
Actions of most of the geomorphic processes vigorous and youthful rivers flowing over steep
and agents are slow, and hence the results gradients. With time, stream channels over
take a long time to take shape. Every landform steep gradients turn gentler due to continued
has a beginning. Landforms once formed may erosion, and as a consequence, lose their
change in their shape, size and nature slowly velocity, facilitating active deposition. There
or fast due to continued action of geomorphic may be depositional forms associated with
processes and agents. streams flowing over steep slopes. But these
Due to changes in climatic conditions and phenomena will be on a small scale compared
vertical or horizontal movements of land- to those associated with rivers flowing over
masses, either the intensity of processes or the medium to gentle slopes. The gentler the river
processes themselves might change leading to channels in gradient or slope, the greater is
new modifications in the landforms. Evolution the deposition. When the stream beds turn
here implies stages of transformation of either gentler due to continued erosion, downward
2020-2156 FUNDAMENTALS OF PHYSICAL GEOGRAPHY
cutting becomes less dominant and lateral Mature
erosion of banks increases and as a
During this stage streams are plenty with good
consequence the hills and valleys are reduced
integration. The valleys are still V-shaped but
to plains.
deep; trunk streams are broad enough to have
wider floodplains within which streams may
Is complete reduction of relief of a high
flow in meanders confined within the valley.
land mass possible?
The flat and broad inter stream areas and
swamps and marshes of youth disappear and
O verland flow causes sheet erosion.
the stream divides turn sharp. Waterfalls and
Depending upon irregularities of the land
rapids disappear.
surface, the overland flow may concentrate into
narrow to wide paths. Because of the sheer
Old
friction of the column of flowing water, minor
or major quantities of materials from the Smaller tributaries during old age are few
surface of the land are removed in the direction with gentle gradients. Streams meander freely
of flow and gradually small and narrow rills over vast floodplains showing natural levees,
will form. These rills will gradually develop into oxbow lakes, etc. Divides are broad and flat
long and wide gullies; the gullies will further with lakes, swamps and marshes. Most of
deepen, widen, lengthen and unite to give the landscape is at or slightly above sea level.
rise to a network of valleys. In the early stages,
down-cutting dominates during which EROSIONAL LANDFORMS
irregularities such as waterfalls and cascades
will be removed. In the middle stages, streams
Valleys
cut their beds slower, and lateral erosion of
valley sides becomes severe. Gradually, the Valleys start as small and narrow rills; the
valley sides are reduced to lower and lower rills will gradually develop into long and wide
slopes. The divides between drainage basins gullies; the gullies will further deepen, widen
are likewise lowered until they are almost and lengthen to give rise to valleys. Depending
completely flattened leaving finally, a lowland upon dimensions and shape, many types of
of faint relief with some low resistant remnants valleys like V-shaped valley, gorge, canyon,
called monadnocks standing out here and etc. can be recognised. A gorge is a deep valley
there. This type of plain forming as a result of with very steep to straight sides (Figure 7.1)
stream erosion is called a peneplain (an almost and a canyon is characterised by steep step-
plain). The characteristics of each of the stages like side slopes (Figure 7.2) and may be as
of landscapes developing in running water deep as a gorge. A gorge is almost equal in
regimes may be summarised as follows: width at its top as well as its bottom. In
contrast, a canyon is wider at its top than at
Youth its bottom. In fact, a canyon is a variant of
Streams are few during this stage with poor gorge. Valley types depend upon the type and
integration and flow over original slopes structure of rocks in which they form. For
showing shallow V-shaped valleys with no example, canyons commonly form in
floodplains or with very narrow floodplains horizontal bedded sedimentary rocks and
along trunk streams. Streams divides are gorges form in hard rocks.
broad and flat with marshes, swamp and
Potholes and Plunge Pools
lakes. Meanders if present develop over these
broad upland surfaces. These meanders may Over the rocky beds of hill-streams more or
eventually entrench themselves into the less circular depressions called potholes form
uplands. Waterfalls and rapids may exist where because of stream erosion aided by the
local hard rock bodies are exposed. abrasion of rock fragments. Once a small and
2020-21LANDFORMS AND THEIR EVOLUTION 57
waterfalls also, large potholes, quite deep and
wide, form because of the sheer impact of
water and rotation of boulders. Such large
and deep holes at the base of waterfalls are
called plunge pools.
Incised or Entrenched Meanders
In streams that flow rapidly over steep
gradients, normally erosion is concentrated
on the bottom of the stream channel. Also, in
the case of steep gradient streams, lateral
erosion on the sides of the valleys is not much
when compared to the streams flowing on
low and gentle slopes. Because of active lateral
erosion, streams flowing over gentle slopes,
develop sinuous or meandering courses. It is
common to find meandering courses over
floodplains and delta plains where stream
gradients are very gentle. But very deep and
wide meanders can also be found cut in hard
rocks. Such meanders are called incised or
entrenched meanders (Figure 7.2).
River Terraces
River terraces are surfaces marking old valley
floor or floodplain levels. They may be bedrock
Figure 7.1 : The Valley of Kaveri river near Hogenekal, surfaces without any alluvial cover or alluvial
Dharmapuri district, Tamil Nadu in the form of gorge terraces consisting of stream deposits. River
terraces are basically products of erosion as
they result due to vertical erosion by the
stream into its own depositional floodplain.
There can be a number of such terraces at
different heights indicating former river bed
levels. The river terraces may occur at the
same elevation on either side of the rivers in
which case they are called paired terraces..
DEPOSITIONAL LANDFORMS
Figure 7.2 : An entrenched meander loop of river Colorado Alluvial Fans
in USA showing step-like side slopes of its valley
typical of a canyon Alluvial fans (Figure 7.4) are formed when
streams flowing from higher levels break into
shallow depression forms, pebbles and foot slope plains of low gradient. Normally
boulders get collected in those depressions and very coarse load is carried by streams flowing
get rotated by flowing water and consequently over mountain slopes. This load becomes too
the depressions grow in dimensions. A series heavy for the streams to be carried over
of such depressions eventually join and the gentler gradients and gets dumped and
stream valley gets deepened. At the foot of spread as a broad low to high cone shaped
2020-2158 FUNDAMENTALS OF PHYSICAL GEOGRAPHY
deposit called alluvial fan. Usually, the streams accumulates as a low cone. Unlike in alluvial
which flow over fans are not confined to their fans, the deposits making up deltas are very
original channels for long and shift their well sorted with clear stratification. The
position across the fan forming many coarsest materials settle out first and the finer
channels called distributaries. Alluvial fans fractions like silts and clays are carried out
in humid areas show normally low cones with into the sea. As the delta grows, the river
gentle slope from head to toe and they appear distributaries continue to increase in length
as high cones with steep slope in arid and (Figure 7.5) and delta continues to build up
semi-arid climates. into the sea.
Floodplains, Natural Levees and Point Bars
Deposition develops a floodplain just as
erosion makes valleys. Floodplain is a major
landform of river deposition. Large sized
materials are deposited first when stream
channel breaks into a gentle slope. Thus,
normally, fine sized materials like sand, silt
and clay are carried by relatively slow
moving waters in gentler channels usually
found in the plains and deposited over the
bed and when the waters spill over the
Figure 7.4 : An alluvial fan deposited by a hill stream
on the way to Amarnath, Jammu and Kashmir banks during flooding above the bed.
A river bed made of river deposits is the active
floodplain. The floodplain above the bank is
Deltas inactive floodplain. Inactive floodplain above
Deltas are like alluvial fans but develop at a the banks basically contain two types of
different location. The load carried by the deposits — flood deposits and channel
rivers is dumped and spread into the sea. If deposits. In plains, channels shift laterally
this load is not carried away far into the sea and change their courses occasionally leaving
or distributed along the coast, it spreads and cut-off courses which get filled up gradually.
Such areas over flood plains built up by
abandoned or cut-off channels contain coarse
deposits. The flood deposits of spilled waters
carry relatively finer materials like silt and
clay. The flood plains in a delta are called
delta plains.
Figure 7.5 : A satellite view of part of Krishna river
delta, Andhra Pradesh Figure 7.6 : Natural levee and point bars
2020-21LANDFORMS AND THEIR EVOLUTION 59
Natural levees and point bars (Figure 7.6)
are some of the important landforms found
associated with floodplains. Natural levees are
found along the banks of large rivers. They
are low, linear and parallel ridges of coarse
deposits along the banks of rivers, quite often
cut into individual mounds. Point bars are
also known as meander bars. They are found
on the concave side of meanders of large rivers
and are sediments deposited in a linear
fashion by flowing waters along the bank.
They are almost uniform in profile and in width
and contain mixed sizes of sediments.
In what way do natural levees differ from
point bars?
Meanders
In large flood and delta plains, rivers rarely
flow in straight courses. Loop-like channel
patterns called meanders develop over flood
and delta plains (Figure 7.7).
Figure 7.8 : Meander growth and cut-off loops and
slip-off and undercut banks
the banks slowly get transformed into a small
curvature in the banks; the curvature
deepens due to deposition on the inside of
the curve and erosion along the bank on the
outside. If there is no deposition and no
erosion or undercutting, the tendency to
meander is reduced. Normally, in meanders
of large rivers, there is active deposition along
Figure 7.7 : A satellite scene showing meandering
Burhi Gandak river near Muzaffarpur, Bihar, showing
the concave bank and undercutting along the
a number of oxbow lakes and cut-offs convex bank. The concave bank is known as
cut-off bank which shows up as a steep scarp
Meander is not a landform but is only a and the convex bank presents a long, gentle
type of channel pattern. This is because of
profile (Figure 7.8). As meanders grow into
(i) propensity of water flowing over very gentle deep loops, the same may get cut-off due to
gradients to work laterally on the banks; erosion at the inflection points and are left
(ii) unconsolidated nature of alluvial deposits as ox-bow lakes.
making up the banks with many irregularities
which can be used by water exerting pressure GROUNDWATER
laterally; (iii) coriolis force acting on the fluid
water deflecting it like it deflects the wind. Here the interest is not on groundwater as a
When the gradient of the channel becomes resource. Our focus is on the work of
extremely low, water flows leisurely and starts groundwater in the erosion of landmasses and
working laterally. Slight irregularities along evolution of landforms. The surface water
2020-2160 FUNDAMENTALS OF PHYSICAL GEOGRAPHY
Figure 7.10 : Various karst features
percolates well when the rocks are permeable, produced by the action of groundwater
thinly bedded and highly jointed and cracked. through the processes of solution and
After vertically going down to some depth, deposition is called Karst topography after the
the water under the ground flows horizontally typical topography developed in limestone
through the bedding planes, joints or through rocks of Karst region in the Balkans adjacent
the materials themselves. It is this downward to Adriatic sea.
and horizontal movement of water which The karst topography is also characterised
causes the rocks to erode. Physical or by erosional and depositional landforms.
mechanical removal of materials by moving
groundwater is insignificant in developing EROSIONAL LANDFORMS
landforms. That is why, the results of the work
of groundwater cannot be seen in all types of Pools, Sinkholes, Lapies and
rocks. But in rocks like limestones or Limestone Pavements
dolomites rich in calcium carbonate, the
surface water as well as groundwater through Small to medium sized round to sub-rounded
the chemical process of solution and shallow depressions called swallow holes form
precipitation deposition develop varieties of on the surface of limestones through solution.
landforms. These two processes of solution Sinkholes are very common in limestone/
and precipitation are active in limestones or karst areas. A sinkhole is an opening more or
dolomites occurring either exclusively or less circular at the top and funnel-shapped
interbedded with other rocks. Any limestone towards the bottom with sizes varying in area
or dolomitic region showing typical landforms from a few sq. m to a hectare and with depth
2020-21LANDFORMS AND THEIR EVOLUTION 61
from a less than half a metre to thirty metres Depositional Landforms
or more. Some of these form solely through
Many depositional forms develop within the
solution action (solution sinks) and others
limestone caves. The chief chemical in
might start as solution forms first and if the limestone is calcium carbonate which is easily
bottom of a sinkhole forms the roof of a void soluble in carbonated water (carbon dioxide
or cave underground, it might collapse leaving absorbed rainwater). This calcium carbonate
a large hole opening into a cave or a void is deposited when the water carrying it in
below (collapse sinks). Quite often, sinkholes solution evaporates or loses its carbon dioxide
are covered up with soil mantle and appear as it trickles over rough rock surfaces.
as shallow water pools. Anybody stepping
over such pools would go down like it Stalactites, Stalagmites and Pillars
happens in quicksands in deserts. The term
doline is sometimes used to refer the collapse Stalactites hang as icicles of dif ferent
sinks. Solution sinks are more common than diameters. Normally they are broad at their
collapse sinks. Quite often the surface run- bases and taper towards the free ends
off simply goes down swallow and sink holes showing up in a variety of forms. Stalagmites
and flow as underground streams and re- rise up from the floor of the caves. In fact,
emerge at a distance downstream through a stalagmites form due to dripping water from
cave opening. When sink holes and dolines the surface or through the thin pipe, of the
join together because of slumping of materials stalactite, immediately below it (Figure 7.11).
along their margins or due to roof collapse of Stalagmites may take the shape of a
caves, long, narrow to wide trenches called column, a disc, with either a smooth, rounded
valley sinks or Uvalas form. Gradually, most bulging end or a miniature crater like
of the surface of the limestone is eaten away
by these pits and trenches, leaving it extremely
irregular with a maze of points, grooves and
ridges or lapies. Especially, these ridges or
lapies form due to differential solution activity
along parallel to sub-parallel joints. The lapie
field may eventually turn into somewhat
smooth limestone pavements.
Caves
In areas where there are alternating beds of
rocks (shales, sandstones, quartzites) with
limestones or dolomites in between or in areas
where limestones are dense, massive and
occurring as thick beds, cave formation is
prominent. Water percolates down either
through the materials or through cracks and
joints and moves horizontally along bedding
planes. It is along these bedding planes that
the limestone dissolves and long and narrow
to wide gaps called caves result. There can
be a maze of caves at different elevations
depending upon the limestone beds and
intervening rocks. Caves normally have an
opening through which cave streams are
discharged. Caves having openings at both
the ends are called tunnels. Figure 7.11 : Stalactites and stalagmites in limestone caves
2020-2162 FUNDAMENTALS OF PHYSICAL GEOGRAPHY
depression. The stalagmite and stalactites fragments) get dragged along the floors or
eventually fuse to give rise to columns and sides of the valleys and cause great damage
pillars of different diameters. through abrasion and plucking. Glaciers can
cause significant damage to even un-
GLACIERS weathered rocks and can reduce high
mountains into low hills and plains.
Masses of ice moving as sheets over the land
As glaciers continue to move, debris gets
(continental glacier or piedmont glacier if a
removed, divides get lowered and eventually
vast sheet of ice is spread over the plains at
the slope is reduced to such an extent that
the foot of mountains) or as linear flows down
glaciers will stop moving leaving only a mass
the slopes of mountains in broad trough-like
of low hills and vast outwash plains along
valleys (mountain and valley glaciers) are
with other depositional features. Figures 7.13
called glaciers (Figure 7.12). The movement
and 7.14 show various glacial erosional and
of glaciers is slow unlike water flow. The
depositional forms described in the text.
movement could be a few centimetres to a
few metres a day or even less or more. Glaciers
EROSIONAL LANDFORMS
move basically because of the force of gravity.
Cirque
Cirques are the most common of landforms
in glaciated mountains. The cirques quite
often are found at the heads of glacial valleys.
The accumulated ice cuts these cirques while
moving down the mountain tops. They are
deep, long and wide troughs or basins with
very steep concave to vertically dropping high
walls at its head as well as sides. A lake of
water can be seen quite often within the
cirques after the glacier disappears. Such
lakes are called cirque or tarn lakes. There
Figure 7.12 : A glacier in its valley can be two or more cirques one leading into
another down below in a stepped sequence.
We have many glaciers in our country
Horns and Serrated Ridges
moving down the slopes and valleys in
Himalayas. Higher reaches of Uttaranchal, Horns form through head ward erosion of
H imachal Pradesh and Jammu and the cirque walls. If three or more radiating
Kashmir, are places to see some of them. glaciers cut headward until their cirques meet,
Do you know where one can see river high, sharp pointed and steep sided peaks
Bhagirathi is basically fed by meltwaters
called horns form. The divides between cirque
from under the snout (Gaumukh) of the
side walls or head walls get narrow because
Gangotri glacier. In fact, Alkapuri glacier
feeds waters to Alakananda river. Rivers
of progressive erosion and turn into serrated
Alkananda and Bhagirathi join to make or saw-toothed ridges sometimes referred to
river Ganga near Deoprayag. as arêtes with very sharp crest and a zig-zag
outline.
Erosion by glaciers is tremendous because The highest peak in the Alps, Matterhorn
of friction caused by sheer weight of the ice. and the highest peak in the Himalayas,
The material plucked from the land by glaciers Everest are in fact horns formed through
(usually large-sized angular blocks and headward erosion of radiating cirques.
2020-21LANDFORMS AND THEIR EVOLUTION 63
Figure 7.13 : Some glacial erosional and depositional forms (adapted and modified from Spencer, 1962)
Glacial Valleys/Troughs Depositional Landforms
Glaciated valleys are trough-like and U - The unassorted coarse and fine debris
shaped with broad floors and relatively dropped by the melting glaciers is called glacial
smooth, and steep sides. The valleys may till. Most of the rock fragments in till are
contain littered debris or debris shaped as angular to sub-angular in form. Streams form
moraines with swampy appearance. There by melting ice at the bottom, sides or lower
may be lakes gouged out of rocky floor or ends of glaciers. Some amount of rock debris
formed by debris within the valleys. There small enough to be carried by such melt-
can be hanging valleys at an elevation on one water streams is washed down and deposited.
or both sides of the main glacial valley. The Such glacio-fluvial deposits are called
faces of divides or spurs of such hanging outwash deposits. Unlike till deposits, the
valleys opening into main glacial valleys are outwash deposits are roughly stratified and
quite often truncated to give them an assorted. The rock fragments in outwash
appearance like triangular facets. Very deep deposits are somewhat rounded at their edges.
glacial troughs filled with sea water and Figure 7.14 shows a few depositional
making up shorelines (in high latitudes) are landforms commonly found in glaciated areas.
called fjords/fiords.
Moraines
What are the basic differences between
They are long ridges of deposits of glacial till.
glacial valleys and river valleys?
Terminal moraines are long ridges of debris
2020-2164 FUNDAMENTALS OF PHYSICAL GEOGRAPHY
Figure 7.14 : A panoramic diagram of glacial landscape with various depositional landforms
(adapted and modified from Spencer, 1962)
deposited at the end (toe) of the glaciers. over the ground (not in a valley cut in the
Lateral moraines form along the sides parallel ground) with ice forming its banks. Very
to the glacial valleys. The lateral moraines may coarse materials like boulders and blocks
join a terminal moraine forming a horse-shoe along with some minor fractions of rock debris
shaped ridge (Fig. 7.13). There can be many carried into this stream settle in the valley of
lateral moraines on either side in a glacial valley. ice beneath the glacier and after the ice melts
These moraines partly or fully owe their origin can be found as a sinuous ridge called esker.
to glacio-fluvial waters pushing up materials
to the sides of glaciers. Many valley glaciers Outwash Plains
retreating rapidly leave an irregular sheet of
The plains at the foot of the glacial mountains
till over their valley floors. Such deposits varying
or beyond the limits of continental ice sheets
greatly in thickness and in surface topography
are covered with glacio-fluvial deposits in the
are called ground moraines. The moraine in the
form of broad flat alluvial fans which may
centre of the glacial valley flanked by lateral
join to form outwash plains of gravel, silt,
moraines is called medial moraine. They are
sand and clay.
imperfectly formed as compared to lateral
moraines. Sometimes medial moraines are Distinguish between river alluvial plains
indistinguishable from ground moraines. and glacial outwash plains.
Eskers
Drumlins
When glaciers melt in summer, the water
flows on the surface of the ice or seeps down Drumlins are smooth oval shaped ridge-like
along the margins or even moves through features composed mainly of glacial till with
holes in the ice. These waters accumulate some masses of gravel and sand. The long
beneath the glacier and flow like streams in axes of drumlins are parallel to the direction
a channel beneath the ice. Such streams flow of ice movement. They may measure up to 1
2020-21LANDFORMS AND THEIR EVOLUTION 65
km in length and 30 m or so in height. One and gently sloping sedimentary coasts
end of the drumlins facing the glacier called (emerged coasts).
the stoss end is blunter and steeper than the
other end called tail. The drumlins form due HIGH ROCKY COASTS
to dumping of rock debris beneath heavily Along the high rocky coasts, the rivers appear
loaded ice through fissures in the glacier. The to have been drowned with highly irregular
stoss end gets blunted due to pushing by coastline. The coastline appears highly indented
moving ice. Drumlins give an indication of
with extension of water into the land where
direction of glacier movement.
glacial valleys (fjords) are present. The hill sides
drop off sharply into the water. Shores do not
What is the difference between till and
show any depositional landforms initially.
alluvium?
Erosion features dominate.
Along high rocky coasts, waves break with
WAVES AND CURRENTS great force against the land shaping the hill
sides into cliffs. With constant pounding by
Coastal processes are the most dynamic and waves, the cliffs recede leaving a wave-cut
hence most destructive. So, don’t you think platform in front of the sea cliff. Waves
it is important to know about the coastal gradually minimise the irregularities along
processes and forms? the shore.
Some of the changes along the coasts take The materials which fall off, and removed
place very fast. At one place, there can be from the sea cliffs, gradually break into
erosion in one season and deposition in smaller fragments and roll to roundness, will
another. Most of the changes along the coasts get deposited in the of fshore. A fter a
are accomplished by waves. When waves considerable period of cliff development and
break, the water is thrown with great force retreat when coastline turns somewhat
onto the shore, and simultaneously, there is smooth, with the addition of some more
a great churning of sediments on the sea material to this deposit in the offshore, a
bottom. Constant impact of breaking waves wave-built terrace would develop in front of
drastically affects the coasts. Storm waves wave-cut terrace. As the erosion along the
and tsunami waves can cause far-reaching coast takes place a good supply material
changes in a short period of time than normal becomes available to longshore currents and
breaking waves. A s wave environment waves to deposit them as beaches along the
changes, the intensity of the force of breaking shore and as bars (long ridges of sand and/
waves changes. or shingle parallel to the coast) in the nearshore
zone. Bars are submerged features and when
Do you know about the generating forces
bars show up above water, they are called
behind waves and currents? If not, refer
barrier bars. Barrier bar which get keyed up
to the chapter on movements in ocean
to the headland of a bay is called a spit. When
waters.
barrier bars and spits form at the mouth of a
bay and block it, a lagoon forms. The lagoons
Other than the action of waves, the coastal
would gradually get filled up by sediments
landforms depend upon (i) the configuration
from the land giving rise to a coastal plain.
of land and sea floor; (ii) whether the coast is
advancing (emerging) seaward or retreating LOW SEDIMENTARY COASTS
(submerging) landward. Assuming sea level
to be constant, two types of coasts A long low sedimentary coasts the rivers
are considered to explain the concept of appear to extend their length by building
evolution of coastal landforms: (i) high, rocky coastal plains and deltas. The coastline
coasts (submerged coasts); (ii) low, smooth appears smooth with occasional incursions
2020-2166 FUNDAMENTALS OF PHYSICAL GEOGRAPHY
of water in the form of lagoons and tidal some remnants of rock standing isolated as
creeks. The land slopes gently into the water. small islands just off the shore. Such resistant
Marshes and swamps may abound along the masses of rock, originally parts of a cliff or
coasts. Depositional features dominate. hill are called sea stacks. Like all other
When waves break over a gently sloping features, sea stacks are also temporary and
sedimentary coast, the bottom sediments get eventually coastal hills and cliffs will disappear
churned and move readily building bars, because of wave erosion giving rise to narrow
barrier bars, spits and lagoons. Lagoons coastal plains, and with onrush of deposits
would eventually turn into a swamp which from over the land behind may get covered
would subsequently turn into a coastal plain. up by alluvium or may get covered up by
The maintenance of these depositional features shingle or sand to form a wide beach.
depends upon the steady supply of materials.
Storm and tsunami waves cause drastic DEPOSITIONAL LANDFORMS
changes irrespective of supply of sediments.
Large rivers which bring lots of sediments Beaches and Dunes
build deltas along low sedimentary coasts. Beaches are characteristic of shorelines that
are dominated by deposition, but may occur
The west coast of our country is a high as patches along even the rugged shores. Most
rocky retreating coast. Erosional forms of the sediment making up the beaches comes
dominate in the west coast. The east from land carried by the streams and rivers
coast of India is a low sedimentary coast. or from wave erosion. Beaches are temporary
Depositional forms dominate in the east features. The sandy beach which appears so
coast. permanent may be reduced to a very narrow
strip of coarse pebbles in some other season.
Most of the beaches are made up of sand
What are the various dif ferences sized materials. Beaches called shingle
between a high rocky coast and a low beaches contain excessively small pebbles and
sedimentary coast in terms of processes even cobbles.
and landforms? Just behind the beach, the sands lifted and
winnowed from over the beach surfaces
EROSIONAL LANDFORMS will be deposited as sand dunes. Sand dunes
forming long ridges parallel to the coastline are
Cliffs, Terraces, Caves and Stacks very common along low sedimentary coasts.
Wave-cut cliffs and terraces are two forms Bars, Barriers and Spits
usually found where erosion is the dominant
shore process. Almost all sea cliffs are steep A ridge of sand and shingle formed in the sea
and may range from a few m to 30 m or even in the off-shore zone (from the position of
more. At the foot of such cliffs there may be low tide waterline to seaward) lying
a flat or gently sloping platform covered by approximately parallel to the coast is called
rock debris derived from the sea cliff behind. an off-shore bar. A n off-shore bar which is
Such platforms occurring at elevations above exposed due to further addition of sand is
the average height of waves is called a wave- termed a barrier bar. The off-shore bars and
cut terrace. The lashing of waves against the barriers commonly form across the mouth of
base of the cliff and the rock debris that gets a river or at the entrance of a bay. Sometimes
smashed against the cliff along with lashing such barrier bars get keyed up to one end of
waves create hollows and these hollows get the bay when they are called spits (Figure
widened and deepened to form sea caves. The 7.15). Spits may also develop attached to
roofs of caves collapse and the sea cliffs recede headlands/hills. The barriers, bars and spits
further inland. Retreat of the cliff may leave at the mouth of the bay gradually extend
2020-21LANDFORMS AND THEIR EVOLUTION 67
are storm winds which are very destructive.
Winds cause deflation, abrasion and impact.
Deflation includes lifting and removal of dust
and smaller particles from the surface of rocks.
In the transportation process sand and silt act
as effective tools to abrade the land surface.
The impact is simply sheer force of momentum
which occurs when sand is blown into or
against a rock surface. It is similar to sand-
blasting operation. The wind action creates a
number of interesting erosional and
depositional features in the deserts.
In fact, many features of deserts owe their
Figure 7.15 : A satellite picture of a part of Godavari
river delta showing a spit formation to mass wasting and running water
as sheet floods. Though rain is scarce in
leaving only a small opening of the bay into deserts, it comes down torrentially in a short
the sea and the bay will eventually develop period of time. The desert rocks devoid of
into a lagoon. The lagoons get filled up vegetation, exposed to mechanical and
gradually by sediment coming from the land chemical weathering processes due to drastic
or from the beach itself (aided by wind) and a diurnal temperature changes, decay faster
broad and wide coastal plain may develop and the torrential rains help in removing the
replacing a lagoon. weathered materials easily. That means, the
weathered debris in deserts is moved by not
Do you know, the coastal off-shore bars only wind but also by rain/sheet wash. The
offer the first buffer or defence against wind moves fine materials and general mass
storm or tsunami by absorbing most of erosion is accomplished mainly through sheet
their destructive force. Then come the floods or sheet wash. Stream channels in
barriers, beaches, beach dunes and desert areas are broad, smooth and indefinite
mangroves, if any, to absorb the and flow for a brief time after rains.
destructive force of storm and tsunami
waves. So, if we do anything which EROSIONAL LANDFORMS
disturbs the ‘sediment budget’ and the
mangroves along the coast, these coastal Pediments and Pediplains
forms will get eroded away leaving human
habitations to bear first strike of storm Landscape evolution in deserts is primarily
and tsunami waves. concerned with the formation and extension
of pediments. Gently inclined rocky floors
close to the mountains at their foot with or
WINDS without a thin cover of debris, are called
Wind is one of the two dominant agents in hot pediments. Such rocky floors form through
deserts. The desert floors get heated up too the erosion of mountain front through a
much and too quickly because of being dry combination of lateral erosion by streams and
and barren. The heated floors heat up the air sheet flooding.
directly above them and result in upward Erosion starts along the steep margins of
movements in the hot lighter air with the landmass or the steep sides of the
turbulence, and any obstructions in its path tectonically controlled steep incision features
sets up eddies, whirlwinds, updrafts and over the landmass. Once, pediments are
downdrafts. Winds also move along the desert formed with a steep wash slope followed by
floors with great speed and the obstructions in cliff or free face above it, the steep wash slope
their path create turbulence. Of course, there and free face retreat backwards. This method
2020-2168 FUNDAMENTALS OF PHYSICAL GEOGRAPHY
of erosion is termed as parallel retreat of slopes
List the erosional features carved out by
through backwasting. So, through parallel wind action and action of sheet floods.
retreat of slopes, the pediments extend
backwards at the expense of mountain front,
and gradually, the mountain gets reduced Depositional Landforms
leaving an inselberg which is a remnant of
Wind is a good sorting agent. Depending upon
the mountain. That’s how the high relief in
the velocity of wind, different sizes of grains
desert areas is reduced to low featureless
are moved along the floors by rolling or
plains called pediplains.
saltation and carried in suspension and in
this process of transportation itself, the
Playas
materials get sorted. When the wind slows
Plains are by far the most prominent or begins to die down, depending upon sizes
landforms in the deserts. In basins with of grains and their critical velocities, the grains
mountains and hills around and along, the will begin to settle. So, in depositional
drainage is towards the centre of the basin landforms made by wind, good sorting of
and due to gradual deposition of sediment grains can be found. Since wind is there
from basin margins, a nearly level plain forms everywhere and wherever there is good source
at the centre of the basin. In times of sufficient of sand and with constant wind directions,
water, this plain is covered up by a shallow depositional features in arid regions can
water body. Such types of shallow lakes are develop anywhere.
called as playas where water is retained only
for short duration due to evaporation and Sand Dunes
quite often the playas contain good deposition
of salts. The playa plain covered up by salts Dry hot deserts are good places for sand dune
is called alkali flats. formation. Obstacles to initiate dune formation
are equally important. There can be a great
Deflation Hollows and Caves variety of dune forms (Figure 7.16).
Weathered mantle from over the rocks or bare
soil, gets blown out by persistent movement
of wind currents in one direction. This process
may create shallow depressions called
deflation hollows. Deflation also creates
numerous small pits or cavities over rock
surfaces. The rock faces suffer impact and
abrasion of wind-borne sand and first
shallow depressions called blow outs are
created, and some of the blow outs become
deeper and wider fit to be called caves.
Mushroom, Table and Pedestal Rocks
Many rock-outcrops in the deserts easily
susceptible to wind deflation and abrasion
are worn out quickly leaving some remnants
of resistant rocks polished beautifully in the
shape of mushroom with a slender stalk and
a broad and rounded pear shaped cap above.
Sometimes, the top surface is broad like a
table top and quite often, the remnants stand
Figure 7.16 : Various types of sand dunes
out like pedestals. Arrows indicate wind direction
2020-21LANDFORMS AND THEIR EVOLUTION 69
Crescent shaped dunes called barchans form when supply of sand is poor and wind
with the points or wings directed away from direction is constant. They appear as long
wind direction i.e., downwind, form where the ridges of considerable length but low in
wind direction is constant and moderate and height. T ransverse dunes are aligned
where the original surface over which sand is perpendicular to wind direction. These dunes
moving is almost uniform. Parabolic dunes form when the wind direction is constant and
form when sandy surfaces are partially covered the source of sand is an elongated feature at
with vegetation. That means parabolic dunes right angles to the wind direction. They may
are reversed barchans with wind direction be very long and low in height. When sand is
being the same. Seif is similar to barchan with plenty, quite often, the regular shaped dunes
a small difference. Seif has only one wing or coalesce and lose their individual
point. This happens when there is shift in characteristics. Most of the dunes in the
wind conditions. The lone wings of seifs can deserts shift and a few of them will get
grow very long and high. Longitudinal dunes stabilised especially near human habitations.
EXERCISES
1. Multiple choice questions.
(i) In which of the following stages of landform development, downward cutting
is dominated?
(a) Youth stage (c) Early mature stage
(b) Late mature stage (d) Old stage
(ii) A deep valley characterised by steep step-like side slopes is known as
(a) U-shaped valley (c) Blind valley
(b) Gorge (d) Canyon
(iii) In which one of the following regions the chemical weathering process is
more dominant than the mechanical process?
(a) Humid region (c) Arid region
(b) Limestone region (d) Glacier region
(iv) Which one of the following sentences best defines the term ‘Lapies’ ?
(a) A small to medium sized shallow depression
(b) A landform whose opening is more or less circular at the top and
funnel shaped towards bottom
(c) A landform formed due to dripping water from surface
(d) An irregular surface with sharp pinnacles, grooves and ridges
(v) A deep, long and wide trough or basin with very steep concave high walls
at its head as well as in sides is known as:
(a) Cirque (c) Lateral Moraine
(b) Glacial valley (d) Esker
2. Answer the following questions in about 30 words.
(i) What do incised meanders in rocks and meanders in plains of alluvium
indicate?
2020-2170 FUNDAMENTALS OF PHYSICAL GEOGRAPHY
(ii) Explain the evolution of valley sinks or uvalas.
(iii) Underground flow of water is more common than surface run-off in
limestone areas. Why?
(iv) Glacial valleys show up many linear depositional forms. Give their
locations and names.
(v) How does wind perform its task in desert areas? Is it the only agent
responsible for the erosional features in the deserts?
3. Answer the following questions in about 150 words.
(i) Running water is by far the most dominating geomorphic agent in shaping
the earth’s surface in humid as well as in arid climates. Explain.
(ii) Limestones behave differently in humid and arid climates. Why? What is
the dominant and almost exclusive geomorphic process in limestone areas
and what are its results?
(iii) How do glaciers accomplish the work of reducing high mountains into low
hills and plains?
Project Work
Identify the landforms, materials and processes around your area.
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